The goal of this research is to analyze in detail how life cycle distributions contribute to understanding various factors of importance in radiobiology. The importance of knowing this is that many agents cause delays or blocks in different parts of the cell cycle. Furthermore, many agents are more effectively in one phase than in others. The first area of study is hyperthermia and particularly thermotolerance. Thermotolerance at 42.0 degrees centigrade as well as split dose heating at 45 degrees centrigrade, will be investigated using flow cytometry to monitor changes in the life cycle distribution. This will be done for CHO, V79, and HeLa cells. Further work will be to study the effects of pH on both survival and cell cycle kinetics when combined with hyperthermia. Finally, hypoxia will be studied to analyze changes in the life cycle distribution and to determine how that may influence killing by hyperthermia.